1. Chapter 12
Section 1

2. Pressure 1
What is pressure?
Pressure is the force per unit area that is applied on the surface of an object.
When your weight is distributed over a larger area, the pressure you exert on the area decreases.

3. Pressure 1
Calculating Pressure
Pressure increases if the force applied increases, and decreases if the area of contact increases.
Pressure can be calculated from this formula.

4. Pressure 1
Calculating Pressure
The unit of pressure in the SI system is the pascal, abbreviated Pa.
One pascal is equal to a force of 1 N applied over an area of 1 m2, or 1 Pa = 1 N/m2.
Because 1 Pa is a small unit of pressure, pressure sometimes is expressed in units of kPa, which is 1,000 Pa.

5. Pressure 1
Pressure and Weight
To calculate the pressure that is exerted on a surface, you need to know the force and the area over which it is applied.
Sometimes the force that is exerted is the weight of an object.

6. Pressure 1
Pressure and Weight
Suppose you are holding a 2-kg book in the palm of your hand.
To find out how much pressure is being exerted on your hand, you first must know the force that the book is exerting on your hand—its weight.

7. Pressure 1
Pressure and Weight
If the area of contact between your hand and the book is 0.003m2, the pressure that is exerted on your hand by the book is:

8. Pressure 1
Pressure and Area
One way to change the pressure that is exerted on an object is to change the area over which the force is applied.
When you hit a nail with a hammer, the force you apply is transmitted through the nail from the head to the tip.

9. Pressure 1
Pressure and Area
The tip of the nail comes to a point and is in contact with the wood over a small area.
Because the contact area is so small, the pressure that is exerted by the nail on the wood is large— large enough to push the wood fibers apart.

10. Pressure 1
Fluid
A fluid is any substance that has no definite shape and has the ability to flow.
You might think of a fluid as being a liquid, such as water or motor oil. But gases are also fluids.
Because air can flow and has no definite shape, air is a fluid.

11. Pressure in a Fluid 1 Suppose you placed an empty glass on a table.
The weight of the glass exerts pressure on the table.
If you fill the glass with water, the weight of the water and glass together exert a force on the table. So the pressure exerted on the table increases.

12. Pressure 1
Pressure in a Fluid
Because the water has weight, the water itself also exerts pressure on the bottom of the glass.
This pressure is the weight of the water divided by the area of the glass bottom.
If you pour more water into the glass, the height of the water in the glass increases and the weight of the water increases. As a result, the pressure exerted by the water increases.

13. Pressure and Fluid Height1
Pressure and Fluid Height
When the height of fluid is greater, the pressure at the bottom of the container is greater. This is always true for any fluid or any container.
The greater the height of a fluid above a surface, the greater the pressure exerted by the fluid on that surface.

14. Pressure Increases with Depth1
Pressure Increases with Depth
If you swim underwater, you might notice
that you can feel pressure in your ears.
As the height of the fluid above you increases, the weight of the fluid above you also increases. As a result, the pressure exerted by the fluid increases with depth.

15. Pressure in All Directions1
Pressure in All Directions
If the pressure that is exerted by a fluid is due to the weight of the fluid, is the pressure in a fluid exerted only downward?
The pressure on all objects in a fluid is exerted on all sides, perpendicular to the surface of the object, no matter what its shape.

16. Pressure 1
Atmospheric Pressure
The atmosphere at Earth’s surface is only about one-thousandth as dense as water. However, the thickness of the atmosphere is large enough to exert a large pressure on objects at Earth’s surface.

17. Pressure 1
Atmospheric Pressure
When you are sitting down, the force pushing down on your body due to atmospheric pressure can be equal to the weight of several small cars.

18. Pressure 1
Atmospheric Pressure
Atmospheric pressure is approximately 100,000 Pa at sea level.
Your body is filled with fluids such as blood that also exert pressure.
The pressure exerted outward by the fluids inside your body balances the pressure exerted by the atmosphere.

19. Pressure 1
Going Higher
As you go higher in the atmosphere, atmospheric pressure decreases as the amount of air above you decreases.
The same is true in an ocean, lake, or pond. The water pressure is highest at the ocean floor and decreases as you go upward.

20. Pressure 1
Barometer
An instrument called a barometer is used to measure atmospheric pressure.
The space at the top of the tube is a vacuum.
Atmospheric pressure pushes liquid up a tube.

21. Pressure 1
Barometer
The liquid reaches a height where the pressure at the bottom of the column of liquid balances the pressure of the atmosphere.
As the atmospheric pressure changes, the force pushing on the surface of the liquid changes.
As a result, the height of the liquid in the tube increases as the atmospheric pressure increases.

22. Question 1 1 What is the unit of pressure in the SI system? A. gram
Section Check 1
Question 1
What is the unit of pressure in the SI system?
A. gram
B. liter
C. newton
D. pascal

23. Section Check 1
Answer
The answer is D. One pascal equals the force of 1 N applied over an area of 1 m2.

24. Section Check 1
Question 2
A substance without a definite shape, and with the ability to flow, is known as a _______.
A. fluid
B. liquid
C. plasma
D. water

25. Section Check 1
Answer
The answer is A. Don’t confuse this with a liquid; gases, such as the air you are breathing now, are fluids but not liquids

26. Question 3 1 Pressure in a liquid _______ with depth. A. decreases
Section Check 1
Question 3
Pressure in a liquid _______ with depth.
A. decreases
B. increases
C. remains the same
D. vanishes

27. Section Check 1
Answer
The answer is B. You may have experienced this in a swimming pool. The deeper you dive, the more pressure you feel.

28. Any questions?